The present invention relates to a magnetic brush developing device, more particularly, to a device having a developer tank for accommodating developer therein and a developing roller for a magnetic brush.
A magnetic brush developing device is widely utilized as a device for making visible an electrostatic latent image formed on an image forming member such as a photosensitive drum. The conventional magnetic brush developing device is constructed so that toner and magnetic carrier are mixed in a developer vessel or tank to form developer, the developer is applied to a developing roller consisting of a magnet roller and a sleeve of nonmagnetic material to form a magnetic brush, and the magnetic brush is arranged at a position opposite to the photosensitive drum and slidably contacted onto the surface of the photosensitive drum having the electrostatic latent image formed thereon, to form a toner image. It is necessary for the developer to maintain the toner density and the charge on the toner at given values in order to form an image having suitable density. To this end, in the conventional magnetic brush developing device, a stirring device is accommodated in the developer vessel or tank, and during operation the magnetic carrier and toner are mixed and stirred by the stirring device so as to obtain the required toner density and toner charge.
In the above conventional magnetic brush developing device, however, the developer, the developing roller and the stirring device are accommodated in the developer vessel, so that the capacity of the developing vessel must be large. In high speed printing devices and copying devices, particularly, a great amount of developer must be accommodated in the developer vessel, so that the capacity thereof must be very large. As a result of this, the space around the photosensitive drum cannot be utilized effectively, thereby limiting or decreasing the degree of freedom in design.
Moreover, the developer must be exchanged as the deterioration of the developer proceeds. In the conventional developing device, the developer and magnetic developing roller occupy together a large volume in the developer vessel, so that operations for removing the developer from the developer tank in order to exchange the developer becomes troublesome. Moreover, the developer is scattered out of the vessel during the exchange thereof, resulting in a stain on the floor.
In the conventional developing device, moreover, the mixing and stirring of the developer are effected by rotating a stirring roller having a number of stirring blades provided thereto. Such a stirring mechanism serves only to beat the developer by the stirring blades, so that the portions of the developer located at the bottom and the sides of the developer tank are not moved and remain in place and thus only the portions of the developer around the developing roller are utilized for developing. As a result of this, the whole developer in the developer tank is not utilized effectively. In this case, when a part of the developer in the developer tank becomes deteriorated, the whole developer on the image seems to be deteriorated so that the number of exchanges needed for the developer becomes large.
In the conventional magnetic brush developing device, moreover, a mechanism for maintaining the height of the developer surface in the developer vessel or tank for the developing roller is not provided, so that, for example, during operation, the amount of developer supplied to the developing roller is different at both ends of the developing roller. As a result of this the thickness of the magnetic brush becomes uneven, and thus an image having uniform density cannot be obtained.